1READV(2) Linux Programmer's Manual READV(2)
2
6 readv, writev, preadv, pwritev, preadv2, pwritev2 - read or write data
7 into multiple buffers
8
10 #include <sys/uio.h>
11 ssize_t readv(int fd, const struct iovec *iov, int iovcnt);
13 ssize_t writev(int fd, const struct iovec *iov, int iovcnt);
15 ssize_t preadv(int fd, const struct iovec *iov, int iovcnt,
17 off_t offset);
18 ssize_t pwritev(int fd, const struct iovec *iov, int iovcnt,
20 off_t offset);
21 ssize_t preadv2(int fd, const struct iovec *iov, int iovcnt,
23 off_t offset, int flags);
24 ssize_t pwritev2(int fd, const struct iovec *iov, int iovcnt,
26 off_t offset, int flags);
27 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
29 preadv(), pwritev():
31 Since glibc 2.19:
32 _DEFAULT_SOURCE
33 Glibc 2.19 and earlier:
34 _BSD_SOURCE
35
37 The readv() system call reads iovcnt buffers from the file associated
38 with the file descriptor fd into the buffers described by iov ("scatter
39 input").
40 The writev() system call writes iovcnt buffers of data described by iov
42 to the file associated with the file descriptor fd ("gather output").
43 The pointer iov points to an array of iovec structures, defined in
45 <sys/uio.h> as:
46 struct iovec {
48 void *iov_base; /* Starting address */
49 size_t iov_len; /* Number of bytes to transfer */
50 };
51 The readv() system call works just like read(2) except that multiple
53 buffers are filled.
54 The writev() system call works just like write(2) except that multiple
56 buffers are written out.
57 Buffers are processed in array order. This means that readv() com‐
59 pletely fills iov[0] before proceeding to iov[1], and so on. (If there
60 is insufficient data, then not all buffers pointed to by iov may be
61 filled.) Similarly, writev() writes out the entire contents of iov[0]
62 before proceeding to iov[1], and so on.
63 The data transfers performed by readv() and writev() are atomic: the
65 data written by writev() is written as a single block that is not
66 intermingled with output from writes in other processes (but see
67 pipe(7) for an exception); analogously, readv() is guaranteed to read a
68 contiguous block of data from the file, regardless of read operations
69 performed in other threads or processes that have file descriptors
70 referring to the same open file description (see open(2)).
71 preadv() and pwritev()
73 The preadv() system call combines the functionality of readv() and
74 pread(2). It performs the same task as readv(), but adds a fourth
75 argument, offset, which specifies the file offset at which the input
76 operation is to be performed.
77 The pwritev() system call combines the functionality of writev() and
79 pwrite(2). It performs the same task as writev(), but adds a fourth
80 argument, offset, which specifies the file offset at which the output
81 operation is to be performed.
82 The file offset is not changed by these system calls. The file
84 referred to by fd must be capable of seeking.
85 preadv2() and pwritev2()
87 These system calls are similar to preadv() and pwritev() calls, but add
88 a fifth argument, flags, which modifies the behavior on a per-call
89 basis.
90 Unlike preadv() and pwritev(), if the offset argument is -1, then the
92 current file offset is used and updated.
93 The flags argument contains a bitwise OR of zero or more of the follow‐
95 ing flags:
96 RWF_DSYNC (since Linux 4.7)
98 Provide a per-write equivalent of the O_DSYNC open(2) flag.
99 This flag is meaningful only for pwritev2(), and its effect
100 applies only to the data range written by the system call.
101 RWF_HIPRI (since Linux 4.6)
103 High priority read/write. Allows block-based filesystems to use
104 polling of the device, which provides lower latency, but may use
105 additional resources. (Currently, this feature is usable only
106 on a file descriptor opened using the O_DIRECT flag.)
107 RWF_SYNC (since Linux 4.7)
109 Provide a per-write equivalent of the O_SYNC open(2) flag. This
110 flag is meaningful only for pwritev2(), and its effect applies
111 only to the data range written by the system call.
112 RWF_NOWAIT (since Linux 4.14)
114 Do not wait for data which is not immediately available. If
115 this flag is specified, the preadv2() system call will return
116 instantly if it would have to read data from the backing storage
117 or wait for a lock. If some data was successfully read, it will
118 return the number of bytes read. If no bytes were read, it will
119 return -1 and set errno to EAGAIN. Currently, this flag is
120 meaningful only for preadv2().
121
123 On success, readv(), preadv() and preadv2() return the number of bytes
124 read; writev(), pwritev() and pwritev2() return the number of bytes
125 written.
126 Note that it is not an error for a successful call to transfer fewer
128 bytes than requested (see read(2) and write(2)).
129 On error, -1 is returned, and errno is set appropriately.
131
133 The errors are as given for read(2) and write(2). Furthermore,
134 preadv(), preadv2(), pwritev(), and pwritev2() can also fail for the
135 same reasons as lseek(2). Additionally, the following errors are
136 defined:
137 EINVAL The sum of the iov_len values overflows an ssize_t value.
139 EINVAL The vector count, iovcnt, is less than zero or greater than the
141 permitted maximum.
142 EINVAL An unknown flag is specified in flags.
144
146 preadv() and pwritev() first appeared in Linux 2.6.30; library support
147 was added in glibc 2.10.
148 preadv2() and pwritev2() first appeared in Linux 4.6. Library support
150 was added in glibc 2.26.
151
153 readv(), writev(): POSIX.1-2001, POSIX.1-2008, 4.4BSD (these system
154 calls first appeared in 4.2BSD).
155 preadv(), pwritev(): nonstandard, but present also on the modern BSDs.
157 preadv2(), pwritev2(): nonstandard Linux extension.
159
161 POSIX.1 allows an implementation to place a limit on the number of
162 items that can be passed in iov. An implementation can advertise its
163 limit by defining IOV_MAX in <limits.h> or at run time via the return
164 value from sysconf(_SC_IOV_MAX). On modern Linux systems, the limit is
165 1024. Back in Linux 2.0 days, this limit was 16.
166 C library/kernel differences
168 The raw preadv() and pwritev() system calls have call signatures that
169 differ slightly from that of the corresponding GNU C library wrapper
170 functions shown in the SYNOPSIS. The final argument, offset, is
171 unpacked by the wrapper functions into two arguments in the system
172 calls:
173 unsigned long pos_l, unsigned long pos
175 These arguments contain, respectively, the low order and high order 32
177 bits of offset.
178 Historical C library/kernel differences
180 To deal with the fact that IOV_MAX was so low on early versions of
181 Linux, the glibc wrapper functions for readv() and writev() did some
182 extra work if they detected that the underlying kernel system call
183 failed because this limit was exceeded. In the case of readv(), the
184 wrapper function allocated a temporary buffer large enough for all of
185 the items specified by iov, passed that buffer in a call to read(2),
186 copied data from the buffer to the locations specified by the iov_base
187 fields of the elements of iov, and then freed the buffer. The wrapper
188 function for writev() performed the analogous task using a temporary
189 buffer and a call to write(2).
190 The need for this extra effort in the glibc wrapper functions went away
192 with Linux 2.2 and later. However, glibc continued to provide this
193 behavior until version 2.10. Starting with glibc version 2.9, the
194 wrapper functions provide this behavior only if the library detects
195 that the system is running a Linux kernel older than version 2.6.18 (an
196 arbitrarily selected kernel version). And since glibc 2.20 (which
197 requires a minimum Linux kernel version of 2.6.32), the glibc wrapper
198 functions always just directly invoke the system calls.
199
201 The following code sample demonstrates the use of writev():
202 char *str0 = "hello ";
204 char *str1 = "world\n";
205 struct iovec iov[2];
206 ssize_t nwritten;
207 iov[0].iov_base = str0;
209 iov[0].iov_len = strlen(str0);
210 iov[1].iov_base = str1;
211 iov[1].iov_len = strlen(str1);
212 nwritten = writev(STDOUT_FILENO, iov, 2);
214
216 pread(2), read(2), write(2)
217
219 This page is part of release 4.15 of the Linux man-pages project. A
220 description of the project, information about reporting bugs, and the
221 latest version of this page, can be found at
222 https://www.kernel.org/doc/man-pages/.
223Linux 2017-09-15 READV(2)